Thermodynamic advantages of flash intercooling in natural gas liquefaction’s precooling cycle

Abstract

This paper evaluates flash intercooling (FIC) in the precooling cycle to enhance the liquefaction process efficiency. Utilizing FIC reduces the precooling refrigerant temperature at the compressors inlet from a superheated to saturated vapor state. Both propane and ammonia are examined as precooling refrigerants. Energy and exergy analyses are adopted to evaluate the propane pre-cooled mixed refrigerant (C3MR) and the ammonia pre-cooled mixed refrigerant (NH3MR) process efficiency of eight case studies at two different ambient temperatures of 25 °C and 40 °C. The liquefaction process was modeled to minimize the specific energy consumption (SEC) and determine optimal operating pressure for compressors. The results obtained using FIC show that at the nominal ambient temperature of 25 °C, the SEC of C3MR and NH3MR is reduced by 12.6% and 4.0%, respectively. On a hotter day with an ambient temperature of 40 °C, the FIC enhances the process efficiency at a greater rate in which the NH3MR process SEC reduces by 5.7%, while C3MR process SEC reduces by 5.0%. Moreover, the findings from the exergy analysis reveal that compressors and heat exchangers are two major contributors to exergy destruction which are responsible for 76% of the cycle exergy destruction. Moreover, employing FIC shows that the largest impact in exergy destruction saving occurs for the C3MR at 25 °C by reducing the exergy destruction by 19%, in which combination of compressors and heat exchangers exergy destruction reduces by 121 kJ/kg.LNG.